Well, roughly translated from Greek, "tachy" means "swift" and "meter" is "measure".

Let's say that you start the chronograph when you cross a mile marker in any highway or when 0.1 appears in the partial odometer (trip meter) of your car, and then stop the chrono in the following mile marker or when your car's trip meter reaches 1.0. You will be able to read directly in the tachymeter scale your average speed in that elapsed mile. If you traveled that mile in 30 seconds, you are going to read 120 in the tachy scale.

You can arrive to the same result when you divide 3600 (seconds in an hour) by the same 30 seconds of our example.

It is important to remark that the tachy scale provides "dimensionless" results, or results without a physical unit (sorry for the math digression here), meaning that if the distance measured is one mile, your reading in the scale will be in miles per hour, but if the distance is measured in kilometers, reading will result in kilometers per hour. There is no need to perform a conversion.